Conventionally, as disclosed in, for example, Japanese Patent Application Laid-Open No. 2003-168772, semiconductor devices are known which are each provided with a groove portion or the like for preventing grease from spreading between a base plate and a cooling fin. Generally, thermal conductive grease is applied to a lower surface of a base plate of a power module, and the base plate and the cooling fin are fixed by screws via the grease. When fixed by screws, these parts are pressurized so that the cooling fin and the grease conform to each other, and further clamped and fixed by screws, and therefore the grease spreads in a surface direction of the base plate during fixing. According to the above-described prior art, a grease diffusion preventing portion is provided on at least one of opposing surfaces of the base plate and the cooling fin to avoid the grease from infiltrating into screw holes and this grease diffusion preventing portion can be a groove portion or a protruding portion.